Hereditary Hemochromatosis (HH) is the most common potentially fatal inherited disorder in people of Northern European origin. Approximately 85% of the cases result from a single base-pair mutation, which converts Cys to Tyr in the HFE protein. This mutation causes a recessive disease resulting in increased iron absorption and accumulation of iron in selected tissues. The iron overload damages these organs leading to cirrhosis of the liver, diabetes, cardiomyopathy, and arthritis. Eighteen years after the cloning and identification of HFE, the mechanisms by which HFE affect iron-homeostasis remain to be determined. We hypothesize that HFE has at least two functions. It lowers iron uptake into the body, globally, by increasing the transcription of hepcidin, a hormone that negatively regulates iron efflux out of intestinal epithelial cells into the blood. HFE also negatively regulates the iron levels in the cell, locally. Specific Aim 1 will determine the mechanism(s) by which transferrin mediates the HFE-dependent regulation of hepcidin. Several models of how HFE increases hepcidin transcription will be tested in primary hepatocyte culture, and mice. Specific Aim 2 will determine the mechanism(s) by which HFE regulates the transport of iron into cells. The long-term goal of this work is to understand the function of HFE.